1. 4. The Atom 1) The Thomson model (“plum-pudding” model)
It was known that atoms were electrically neutral, but that they could become charged, implying that there were positive and negative charges and that some of them could be removed.
This model had the atom consisting of a bulk positive charge, with negative electrons buried throughout.
Later, Rutherford did an experiment that showed that the positively charged nucleus must be extremely small compared to the rest of the atom.

2. 2) Rutherford’s scanning experiment and planetary model
Rutherford scattered alpha particles – helium nuclei – from a metal foil and observed the scattering angle. He found that some of the angles were far larger than the plum-pudding model would allow.
Rutherford’s (planetary) model:
The only way to account for the large angles was to assume that all the positive charge was contained within a tiny but massive nucleus.

3. 3) Atomic line spectra (Key to the structure of the atom)
A very thin gas heated in a discharge tube emits light only at characteristic frequencies.
An atomic spectrum is a line spectrum – only certain frequencies appear.
If white light passes through such a gas, it absorbs at those same frequencies.

4. 4) Hydrogen atom
The wavelengths of electrons emitted from hydrogen have a regular pattern:
Rydberg constant:
A portion of the complete spectrum of hydrogen
(The lines cannot be explained by the Rutherford theory)

5. 5) The Bohr Atom
Bohr proposed that the possible energy states (stationary states) for atomic electrons were quantized – only certain values were possible. Then the spectrum could be explained as transitions from one level to another.
Example:

6. An electron is held in orbit by the Coulomb force:
5a) Stable orbits (The Bohr Atom)
Quantization:
An electron is held in orbit by the Coulomb force:

7. The Bohr Atom
The lowest energy level is called the ground state; the others are excited states.

8. Example: Franck- Hertz experiment
Franck and Hertz studied the motion of electrons through mercury vapor under the action of an electric field. When the electron kinetic energy was 4.9eV or grater, the vapor emitted ultraviolet light. What was the wave length of this light?